122 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 









Fig. 7.15. Cardiac muscle. A and B, ventricle of a monkey's heart; C, from a 

 human heart. /, intercalated disks, the cross bands that are characteristic of heart 

 muscle; p, granules of pigment; v, blood capillaries carrying rich supply of blood. 

 (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, 

 McGraw-Hill Book Co., Inc., 1953.) 



conductivity, the ability to transmit the reactions from one place to another. 

 The nerve cell or neuron is the structural unit of the nervous system. Its 

 striking feature is the extension of the cell body into processes. These in- 

 clude two types: the relatively short dendrites through which the changes 

 known as nerve impulses move toward the cell body, and a single process, 

 the axon, through which nerve impulses move away from the cell body 

 (Fig. 7.16). In different parts of the nervous system the cell bodies vary 

 widely in size and shape but all of them have certain characteristics in com- 

 mon. They have prominent nuclei, no centrosomes, fine fibrils which become 

 visible in the cytoplasm with special stains, the neurofibrils, and irregularly 

 shaped bodies, the Nissl or tigroid bodies. The state of the Nissl substance is 

 a sensitive indicator of the condition of the nerve cell. It is depleted in infec- 

 tions such as poliomyelitis, in intoxications, and exhaustion, and is reformed 

 during recovery from illness or during sleep. In all but the simplest animals, 

 such as hydra, the nerve-cell bodies exist only in ganglia and in the gray 

 matter of the brain and spinal cord. 



